1. Field of the Invention
In general, the present invention relates to the structure of heat exchangers. More particularly, the present invention relates to heat exchangers that transfer heat from exhaust gases to the source material used to produce the exhaust gases, thereby preheating the source material.
2. Prior Art Statement
Heat exchangers are typically used to recover heat energy that would otherwise be wasted. Generally, the more energy that is recovered by a system, the more efficient that system can operate. It will therefore be understood that with rising fuel prices and greenhouse gas concerns, the recovery and reuse of exhaust heat energy is particularly important in systems that react hydrocarbon fuels. The more energy that can be recovered and reused, the less hydrocarbon fuel that needs to be consumed in order to generate a given amount of useful energy. Thus, by recovering heat from exhaust gases, the fuel efficiency of the system increases. It will therefore be understood that when designers attempt to make a system more efficient a heat exchanger is often used to recover and reuse heat energy. It will further be understood that the more efficient a heat exchanger system can be made, the more fuel efficient the overall system can be made.
In an attempt to make heat exchanger systems highly efficient, extremely complex multi-stage heat exchanger systems have been designed and integrated devices like automobile engines. Consider, by way of example, U.S. Pat. No. 7,069,977 to Shinohara, entitled Heat Exchanger. Although such heat exchangers do increase the efficiency of certain systems, the application of such heat exchangers is limited to scenarios where a significant temperature differential exists between the materials across which heat is being exchanged.
There are many systems, other than internal combustion engines, that react gases in order to obtain some useful byproduct. For instance, many polymers are created by reacting different vapors. In certain fuel cell systems, water vapor and fuel vapor are reacted to create the hydrogen gas used to power the fuel cell. In such systems, it is often the case that some of the materials to be reacted are liquids at ambient pressure and temperature. This is often the case if the material to be reacted is water, or a distilled petroleum product. Accordingly, the material to be reacted has to be actively heated and converted into a gas before it can be utilized in a reaction. Since the material to be reacted has to be actively heated, the resultant gases exhausted after the desired reaction typically contain a significant amount of heat energy. Much of this heat energy is commonly lost as the resultant gases are vented as exhaust while still containing the latent heat of vaporization.
In order to conserve energy, it makes sense to recapture some of the heat energy present in the exhaust gases and use that heat energy to help vaporize the liquid material that is to be reacted. As long as the temperature of the exhaust gases exceeds the temperature of the liquid material, it is relatively simple to transfer heat energy from exhaust gases to the liquid material. The problem becomes much more complex in systems where the outgoing exhaust gas and incoming liquid material approach a temperature equilibrium. In the case of a two channel steam heat exchanger when the exiting stream is of water vapor is at the boiling temperature and the incoming stream of is water at the boiling temperature, there is no longer any transfer of energy between the two streams even though most of the energy is contained in the exiting stream of vaporized water and relatively little energy in the incoming stream of liquid water. This is because both streams are the same temperature and a temperature difference is required to transfer energy from one stream to the other.
In order for a liquid to be heated into a vapor, it must change phase from a liquid to a gas. In order for the liquid material to change phase, heat energy must be added to the liquid material. Although significant heat energy may be added, the heat energy is utilized in the phase change and does not affect temperature. For example, it takes a significant amount of energy to convert water at 100 degrees Celsius to steam at 100 degrees Celsius at one atmosphere of pressure. It will therefore be understood that steam at 100 degrees Celsius contains the latent heat of vaporization and therefore contains far more energy than does water at 100 degrees Celsius.
Once a material has been vaporized, it is very difficult to recover the latent heat of vaporization using traditional heat exchanger designs. Traditional heat exchangers rely upon a temperature differential to be present between materials in order to transfer heat between those materials. Traditional heat exchangers absorb heat energy from the high temperature material, and using convection and/or conduction, transfer that heat energy to the low temperature material.
In a system where a liquid material is generally at the same temperature as the vaporized material, such as a counter flow heat exchanger were water is the material in both channels, there is no temperature differential when one channel is liquid water and the other channel steam at the same pressure. Although the vaporized material contains the latent heat of vaporization, this energy cannot be recovered because there is no impetus of a temperature differential to cause the heat energy to flow.
A need therefore exists for a heat exchanger system that can effectively transfer energy between a vapor and a liquid even though there is little or no temperature difference between the vapor and the liquid. This need is met by the present invention as described and claimed.